(1) Field of the Invention
The present invention relates to a method and apparatus for feedback control of an air-fuel ratio in an internal combustion engine having two air-fuel ratio sensors upstream and downstream of a catalyst converter disposed within an exhaust gas passage.
(2) Description of the Related Art
Generally, in a feedback control of the air-fuel ratio sensor (O.sub.2 sensor) system, a base fuel amount TAUP is calculated in accordance with the detected intake air amount and detected engine speed, and the base fuel amount TAUP is corrected by an air-fuel ratio correction coefficient FAF which is calculated in accordance with the output of an air-fuel ratio sensor (for example, an O.sub.2 sensor) for detecting the concentration of a specific component such as the oxygen component in the exhaust gas. Thus, an actual fuel amount is controlled in accordance with the corrected fuel amount. The above-mentioned process is repeated so that the air-fuel ratio of the engine is brought close to a stoichiometric air-fuel ratio.
According to this feedback control, the center of the controlled air-fuel ratio can be within a very small range of air-fuel ratios around the stoichiometric ratio required for three-way reducing and oxidizing catalysts (catalyst converter) which can remove three pollutants CO, HC, and NO.sub.X simultaneously from the exhaust gas.
In the above-mentioned O.sub.2 sensor system where the O.sub.2 sensor is disposed at a location near the concentration portion of an exhaust manifold, i.e., upstream of the catalyst converter, the accuracy of the controlled air-fuel ratio is affected by individual differences in the characteristics of the parts of the engine, such as the O.sub.2 sensor, the fuel injection valves, the exhaust gas recirculation (EGR) valve, the valve lifters, individual changes due to the aging of these parts, environmental changes, and the like. That is, if the characteristics of the O.sub.2 sensor fluctuate, or if the uniformity of the exhaust gas fluctuates, the accuracy of the air-fuel ratio feedback correction amount FAF is also fluctuated, thereby causing fluctuations in the controlled air-fuel ratio.
To compensate for the fluctuation of the controlled air-fuel ratio, double O.sub.2 sensor systems have been suggested (see: U.S. Pat. Nos. 3,939,654, 4,027,477, 4,130,095, 4,235,204). In a double O.sub.2 sensor system, another O.sub.2 sensor is provided downstream of the catalyst converter, and thus an air-fuel ratio control operation is carried out by the downstream-side O.sub.2 sensor is addition to an air-fuel ratio control operation carried out by the upstream-side O.sub.2 sensor. In the double O.sub.2 sensor system, although the downstream-side O.sub.2 sensor has lower response speed characteristics when compared with the upstream-side O.sub.2 sensor, the downstream-side O.sub.2 sensor has an advantage in that the output fluctuation characteristics are small when compared with those of the upstream-side O.sub.2 sensor, for the following reasons:
(1) On the downstream side of the catalyst converter, the temperature of the exhaust gas is low, so that the downstream-side O.sub.2 sensor is not affected by a high temperature exhaust gas.
(2) On the downstream side of the catalyst converter, although various kinds of pollutants are trapped in the catalyst converter, these pollutants have little affect on the downstream side O.sub.2 sensor.
(3) On the downstream side of the catalyst converter, the exhaust gas is mixed so that the concentration, of oxygen in the exhaust gas is approximately in an equilibrium state.
Therefore, according to the double O.sub.2 sensor system, the fluctuation of the output of the upstream-side O.sub.2 sensor is compensated for by a feedback control using the output of the downstream-side O.sub.2 sensor. Actually, as illustrated in FIG. 1, in the worst case, the deterioration of the output characteristics of the O.sub.2 sensor in a single O.sub.2 sensor system directly effects a deterioration in the emission characteristics. On the other hand, in a double O.sub.2 sensor system, even when the output characteristics of the upstream-side O.sub.2 sensor are deteriorated, the emission characteristics are not deteriorated. That is, in a double O.sub.2 sensor system, even if only the output characteristics of the downstream-side O.sub.2 are stable, good emission characteristics are still obtained.
In the above-mentioned double O.sub.2 sensor system, however, the air-fuel ratio correction coefficient FAF may be greatly deviated from a reference value such as 1.0 due to individual differences in the characteristics of the parts of the engine, individual changes caused by aging, environmental changes, and the like. For example, when driving at a high altitude (above sea level), the air-fuel ratio correction coefficient FAF is remarkably reduced, thereby obtaining an optimum air-fuel ratio such as the stoichiometric air-fuel ratio. In this case, a maximum value and a minimum value are imposed on the air-fuel ratio correction coefficient FAF, thereby preventing the controlled air-fuel ratio from becoming overrich or overlean. Therefore, when the air-fuel ratio correction coefficient FAF is close to the maximum value or the minimum value, the margin of the air-fuel ratio correction coefficient FAF becomes small, thus limiting the compensation of a transient change of the controlled air-fuel ratio. Also, when the engine is switched from an air-fuel ratio feedback control (closed-loop control) by the upstream-side and downstream-side O.sub.2 sensors to an open-loop control, the air-fuel ratio correction coefficient FAF is made the reference value (=1.0), thereby causing an overrich or overlean condition in the controlled air-fuel ratio, and thus deteriorating the fuel consumption, the drivability, and the condition of the exhaust emissions such as HC, CO, and NO.sub.X, since the air-fuel ratio correction coefficient FAF (=0.1) during an open-loop control is, in this case, not an optimum level. Further, it takes a long time for the controlled air-fuel ratio to reach an optimum level after the engine is switched from an open control to an air-fuel ratio feedback control by the upstream-side and downstream-side O.sub.2 sensors, thus also deteriorating the fuel consumption, the drivability, and the condition of the exhaust emissions.
Accordingly, a learning control operation has been introduced into a double O.sub.2 sensor system, so that a mean value of the air-fuel ratio correction coefficient FAF, i.e., a mean value of successive values of the air-fuel ratio correction coefficient FAF immediately before skip operations is always changed around the reference value such as 1.0. Therefore, the margin of the air-fuel ratio correction coefficient FAF is always large, and accordingly, a transient change in the controlled air-fuel ratio can be compensated. Also, a difference in the air-fuel ratio correction coefficient FAF between an air-fuel ratio feedback control and an open-loop control becomes small. As a result, the deviation of the controlled air-fuel ratio in an open-loop control from its optimum level is small, and in addition, the controlled air-fuel ratio promptly reaches an optimum level after the engine is switched from an open-loop control to an air-fuel ratio feedback control.
In the above-mentioned learning control operation, a learning value FGHAC is calculated so that the mean value FAFAV of the air-fuel ratio correction coefficient FAF is brought close to the reference value such as 1.0. This learning control operation originally responds to a change of density of the air intake into the engine such as when driving at a high altitude. Therefore, a maximum value and a minimum value are also imposed on the learning value FGHAC, thereby preventing the controlled air-fuel ratio from becoming overrich or overlean due to the operation of an evaporation system. In a double O.sub.2 sensor system, however, the base air-fuel ratio is controlled by changing the deviation of the air-fuel ratio correction coefficient FAF from the reference value such as 1.0. Accordingly, since the mean value FAFAV of the air-fuel ratio correction coefficient FAF is changed by the air-fuel ratio feedback control by the downstream-side O.sub.2 sensor even when no change occurs in the intake air density, the learning value FGHAC is changed and brought close to the maximum value or minimum value thereof. Therefore, in this case, the margin of the learning value FGHAC becomes small, and even when a change occurs in the intake air density, compensation of the change of the intake air density may be impossible, thus also deteriorating the fuel consumption, the drivability, and the condition of the exhaust emissions.